The present invention relates to a crane construction and, more particularly, to a transfer crane and driving system therefor.
Cranes are generally provided with traveling wheels in legs formed by frame members and, typically, transfer cranes are provided with traveling wheels generally having rubber tires.
In a crane of the aforementioned type, a crane frame includes a pair of girders, four leg members supporting the girders, right and left saddle members supporting the leg members, and rubber-tired wheels provided at lower parts of the crane frame in such a manner that the weight of the entire crane is usually supported by four rubber tires. Two of the four rubber-tired wheels, located on either side of the crane, are provided with drive or travel motors for dirving the wheels, with the crane being moved by transmission of a rotational power of the drive or travel motor to the rubber-tired wheels. When moving, the right and left travel motors are simultaneously driven and the crane moves along a predetermined path. If the crane deviates from the predetermined path, the speeds of the right and left travel motors are controlled to return the crane to the predetermined path by a so-called positional or steering control. The control of the speed of the drive travel motors to enable the positional or steering control must be very precise and a wide speed control range is essential; therefore, DC motors are used as the travel or drive motors, and a Leonard control system is adopted for the speed control.
A trolley is generally mounted on the girders of the crane, with the trolley being adapted to be moved transversely along the girders by a traverse drive motor. A hoisting accessory such as, for example, a spreader is suspended by the trolley by cables or ropes, with the spreader being adapted to be hoisted up and down by hoist devices which, for example, comprise a drum about which a cable or rope is wound, a hoist motor for driving the drum, and other units. Transfer cranes of the aforementioned type are disclosed, for example, in U.S. Pat. Nos. 4,329,632 and 4,424,875.
As apparent from the above description, a transfer crane is usually provided with at least two DC travel motors, at least one traverse DC motor, and at least one hoist motor which may either be an AC or a DC motor and, by virtue of this fact, a considerable number of controlling devices for the drive motors as well as the power sources, usually generators for supplying power to the motors, are required. Therefore, the entire crane is very large resulting in difficulties in maintenance and inspection thereof in addition to a very high production cost.
The aim underlying the present invention essentially resides in providing a crane and driving system therefor wherein the driving elements of the driving system are of a simplified construction while not in any way effectng or reducing the overall functioning of the crane but which nevertheless enables an easy maintenance and inspection.
In accordance with advantageous features of the present invention, a crane is provided which is equippd with a crane frame having traveling wheels, with one or more AC hoist motors driving a hoist apparatus mounted on the crane frame. One or more AC travel motors are provided for driving the traveling wheels, with a power source generating DC power. At least one inverter unit is provided with the inverter unit inputting the DC power and converting the same into an AC power of a frequency corresponding to a speed instruction and outputting AC power. A switching means is provided selectively supplying the AC power from the at least one inverter unit either to the AC hoist motor or to the AC travel motors.
Advantageously, in accordance with further features of the present invention, the power from the at least one inverter unit is solely supplied to the travel motors during traveling and solely to the hoist motors during the hoisting of an object.
Preferably, according to the present invention, the AC hoist and AC travel motors are induction motors, and the power source is a DC generator driven by, for example, an engine.
One of the AC travel motors is advantageously installed on each side of the crane for driving the traveling wheels provided on the corresonding side of the crane, with the frequencies of the Ac power signal to each of the AC travel motors being altered to effect positional control.
In accordance with yet further features of the present invention, two AC hoist motors are provided, with an electrical magnetic clutch means being provided for selectively driving the AC hoist motors independently or together.
According to the present invention, two inverter units may be provided, with the change over or switch means supplying the AC power output by each of the two inverter units to the travel motor on the corresponding side of the crane when the crane is operated so as to move the same, with the AC power output by each of the inverter units being supplied to the corresponding AC hoist motor when the hoist apparatus is operated.
Traveling wheels are mounted on leg members of the crane frame, with an trolley also being mounted on the crane frame along with a AC traverse motor driving the trolley. The hoist apparatus is provided with hoisting accessories and is vertically movable by the motion of the hoist apparatus. The change over or switch means supplies the AC power to the AC travel motors when the crane is operated so as to move the same, while supplying the AC power to the AC hoist motors when the hoisting apparatus is operated. Inverter units, for converting the DC power into AC power, are controlled so as to alter the frequency of the AC power.
As apparent from the above described features of the present invention, a crane driving system is realized wherein the travel motors for moving the crane and the hoist motors driving the hoisting apparatus provided on the crane are all AC motors, with the motors having common inverter units for converting DC power into AC power, with the AC power being supplied only to the travel motors when the crane is moved and only to the hoist motors when the hoisting apparatus is driven.
It is an object of the present invention to provide a crane and driving system therefor which avoids, by simple means, shortcomings and disadvantages encountered in the prior art.
Another object of the present invention resides in providing a crane and drive system which is simple in construction and therefore relatively inexpensive to manufacture.
Yet another object of the present invention resides in providing a crane drive system therefor which is easy to maintain and inspect.
A still further object of the present invention resides in providing a crane and drive system therefor which functions realiably under all operating conditions.
These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purposes of illustration, one embodiment in accordance with the present invention.